This invention relates to a machining strain removal apparatus for removing machining strain present on a treated surface of a workpiece by polishing the treated surface. More particularly, the invention relates to, but is not limited to, a machining strain removal apparatus suitable for removing machining strain from the back of a semiconductor wafer, which has many circuits formed on the face thereof, by polishing the back of the semiconductor wafer, the machining strain having been generated by grinding.
In a process for production of a semiconductor chip, many rectangular regions are demarcated by streets arranged in a lattice pattern on the face of a semiconductor wafer, and a semiconductor circuit is disposed in each of the rectangular regions. Then, the semiconductor wafer is divided along the streets to form the respective rectangular regions into semiconductor chips. To achieve the compactness and light weight of the semiconductor chips, it is common practice to grind the back of the semiconductor wafer, thereby decreasing the thickness of the semiconductor wafer, before cutting the semiconductor wafer along the streets to separate the rectangular regions individually. In an alternative mode, called the dicing-before-grinding mode, the face of a semiconductor wafer is cut along streets to form grooves of a predetermined depth, and then the back of the semiconductor wafer is ground to a depth exceeding the bottom of the grooves, thereby reducing the thickness of the semiconductor wafer and also separating the rectangular regions individually. Grinding is generally carried out by applying to the back of the semiconductor wafer a rotary grinding tool having a grinding member or grinding wheel formed by binding diamond abrasive grains with a suitable bond such as resin bond.
When the back of the semiconductor wafer is ground, however, machining strain is caused to the back of the semiconductor wafer, and considerably decreases the bending strength of the semiconductor wafer. To remove machining strain from the back of the semiconductor wafer and avoid the decrease in the bending strength, it has been proposed to polish the ground back of the semiconductor wafer with the use of free abrasive grains; to chemically etch the ground back of the semiconductor wafer with the use of an etching solution containing nitric acid and hydrofluoric acid; or to apply a plasma onto the ground back of the semiconductor wafer, thereby etching the back of the semiconductor wafer physically.
The polishing using the free abrasive grains poses the problems that tiresome procedures are necessary for the supply and recovery of the free abrasive grains, resulting in a low efficiency of polishing, and that the free abrasive grains used in large amounts have to be disposed of as an industrial waste. The chemical etching and the physical etching present the problems that considerably expensive equipment is needed, and that it is difficult to apply sufficiently uniform etching.
As disclosed in Japanese Patent Application No. 2001-93397 (Title of the Invention xe2x80x9cPolishing Toolxe2x80x9d) filed by the present applicant, it has been found that machining strain can be removed effectively by polishing the back of a semiconductor wafer with the use of a polishing tool, especially, a polishing tool having a polishing member composed of felt and abrasive grains dispersed in the felt. Polishing, which uses such a polishing tool, is free from the occurrence of a large amount of a waste which has to be disposed of as an industrial waste.
A principal object of the present invention is to provide a novel and excellent machining strain removal apparatus which can remove machining strain present on a treated surface of a workpiece, for example, a ground back of a semiconductor wafer by polishing the treated surface or ground back with a polishing tool with a high efficiency in a high quality.
According to the present invention, there is provided, as a machining strain removal apparatus for attaining the above principal object, a machining strain removal apparatus for removing machining strain present on a treated surface of a workpiece by polishing the treated surface, comprising:
chuck means for holding the workpiece while exposing the treated surface;
workpiece admission/delivery means for admitting the workpiece, in which the machining strain should be removed from the treated surface, onto the chuck means and delivering the workpiece, in which the machining strain has been removed from the treated surface, from a position on the chuck means; and
polishing means for polishing the treated surface of the workpiece held on the chuck means, and wherein
the chuck means is selectively positioned in a workpiece admission/delivery area and a polishing area, and when the chuck means is located in the workpiece admission/delivery area, the workpiece having the machining strain to be removed from the treated surface is admitted onto the chuck means, then the chuck means is moved to the polishing area, and the treated surface of the workpiece held on the chuck means is polished by the polishing means to have the machining strain removed from the treated surface, whereafter the chuck means is returned to the workpiece admission/delivery area, and the workpiece is delivered from the position on the chuck means; and
the polishing means includes a rotating shaft and a polishing tool mounted on the rotating shaft, and the polishing tool being rotated is pressed against the treated surface of the workpiece, whereby the treated surface is polished.
In a preferred embodiment, the workpiece is a semiconductor wafer having many circuits formed on a face thereof, and the treated surface is a ground back of the semiconductor wafer. The polishing tool preferably has a polishing member composed of felt and abrasive grains dispersed in the felt. The polishing tool may have a support member having a circular support surface, and the polishing member may be in the shape of a disk bonded to the circular support surface of the support member. Preferably, the machining strain removal apparatus further comprises dressing means for dressing the polishing member by jetting a high pressure gas at the polishing member, and cooling means for jetting a cooling gas at the polishing tool and/or the workpiece in the polishing area. It is preferred that when the treated surface of the workpiece is polished by the polishing means, the chuck means is rotated about a central axis of rotation extending parallel to the rotating shaft of the polishing means, and is also reciprocated in directions substantially perpendicular to the rotating shaft of the polishing means. Advantageously, the chuck means is movable along a straight path extending in the directions substantially perpendicular to the rotating shaft, and a movement of the chuck means when selectively positioned in the workpiece admission/delivery area and the polishing area and a reciprocating movement of the chuck means during polishing of the treated surface of the workpiece by the polishing means are both along the straight path. Preferably, a dust cover is disposed for surrounding the chuck means located in the polishing area, the workpiece held on the chuck means, and the polishing tool pressed against the treated surface of the workpiece, an opening is formed in the dust cover so as to allow the chuck means and the workpiece held on the chuck means to pass through the opening when the chuck means moves from the workpiece admission/delivery area to the polishing area and when the chuck means moves from the polishing area to the workpiece admission/delivery area, and an exhaust duct for exhausting an interior of the dust cover is connected to the dust cover. Also preferably, the rotating shaft of the polishing means is movable in a direction of a central axis thereof, and an opening is formed in the dust cover so as to allow the polishing tool to pass through the opening when the polishing tool is moved toward and away from the workpiece held on the chuck means by movement of the rotating shaft in the direction of the central axis thereof. It is preferred that the chuck means includes a chuck plate formed from a porous material and having a substantially flat surface, the workpiece is attracted onto the chuck plate, and chuck plate cleaning means is disposed for cleaning the chuck plate. It is also preferred that the chuck plate cleaning means includes a cleaning brush and an oil stone, and the cleaning brush and the oil stone are each pressed against the surface of the chuck plate and are each also rotated about a central axis of rotation extending substantially perpendicularly to the surface of the chuck plate and reciprocated in directions substantially parallel to the surface of the chuck plate.